dm thin: avoid unnecessarily breaking sharing for flushes

There's no need to break sharing, triggering a copy, for a write that has no
data (i.e. a flush).

Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>